The objective of the proposed research is to determine if transovarial transmission of Japanese encephalitis, St. Louis encephalitis, and related viruses by mosquitoes play a role in the survival of these viruses in nature during winter or other periods adverse for their mosquito hosts. It is hypothesized that the viruses survive such periods in eggs of Aedes mosquitoes, and that infection of Culex species (important in transmission of the viruses to man) takes place by feeding on vertebrates infected by Aedes. Field collections of large numbers of immature stages of aedine mosquitoes will be made in Korea, Taiwan, and Hong Kong in areas endemic for Japanese encephalitis virus. Insofar as possible, collections will be made in sites with abundant wild avian populations relatively unaltered by human activity. Pools of the immature mosquitoes will be assayed for the presence of Japanese encephalitis virus by the inoculation of Toxorhynchites amboinensis mosquitoes. The latter will be examined two weeks later by the fluorescent antibody technique. Attempts also will be made to isolate St. Louis encephalitis virus from progeny of various species of Aedes mosquitoes infected in the laboratory by collaborators in California.